1. Field of the Invention
The present invention relates to an electronic endoscopic apparatus that includes an endoscopic scope on which a solid-state imaging device is mounted and an image processing processor performing predetermined image processing on an image signal from the endoscopic scope.
Priority is claimed on Japanese Patent Application No. 2011-136410 filed on Jun. 20, 2011, the content of which is incorporated herein by reference.
2. Description of Related Art
With the recent advancement of semiconductor technology, solid-state imaging devices such as Charge-Coupled Devices (CCD) or Complementary Metal Oxide Semiconductor (CMOS) sensors are moving toward higher pixels.
Electronic endoscopes on which solid-state imaging devices are mounted are no exception to this trend, and the electronic endoscopes are moving toward higher definition.
With the trend toward higher pixels in the solid-state imaging devices, a frequency of a clock signal required for image processing increases as well, and various phenomena take place. For example, in the electronic endoscopes, a distal end of an endoscopic scope on which the solid-state imaging device is mounted is separated from an image processing processor performing image processing. For this reason, a transmission line between the endoscopic scope and the image processing processor may be subjected to signal degradation. When a signal transmitted between the endoscopic scope and the image processing processor increases in frequency, the signal degradation becomes greater. Because of the high-frequency signal traveling through the transmission line, leakage of electromagnetic waves also becomes more prominent.
An electronic endoscopic apparatus is proposed in Japanese Unexamined Patent. Application, First Publication No. 2001-275956. In this electronic endoscopic apparatus, a waveform smoothing circuit is inserted into an output portion of an electronic scope. Due to this waveform smoothing circuit, high-frequency noise released between the electronic scope and a processor device is inhibited.
Japanese Unexamined Patent Application, First Publication No. 2001-275956 contains no teaching in terms of synchronization between an endoscopic scope and a monitor instrument. Since a solid-state imaging device having various angles of view depending on a target to be observed and use is mounted on the endoscopic scope, an operating frequency and an angle of view are different according to the endoscopic scope. Accordingly, to display an image captured by the endoscopic scope on the monitor, frequency conversion adapted to a synchronization signal of the monitor is required.
Depending on a relationship between a display clock and an imaging clock, there is a subtle difference between a cycle in which the endoscopic scope captures an image of one frame and a cycle in which the monitor instrument displays an image of one frame. As such, both cycles are gradually shifted in phase. Further, when the phase shift between the cycles exceeds a time of one frame, a phenomenon called “passing” or “frame dropping” takes place.
FIG. 13 schematically shows a relation between a one-frame cycle based on an imaging clock and a one-frame cycle based on a display clock. As shown in FIG. 13, the one-frame cycle based on the imaging clock and the one-frame cycle based on the display clock are subtly different from each other. Accordingly, a shift D0, D1, D2 of the one-frame cycle increases with the passage of time.